Optical waveguide fiber cables are presently used to connect moving parts of electronic equipment which require data transmission or exchange of information--carying signals between the moving parts. Such equipment as robots, high-speed printer heads, and any equipment that requires parts of it to be constantly or intermittently in motion are useful applications of such coiled cables.
In coiled optical waveguide fiber cables, the integrity of the signals transmitted by the fiber must be maintained while, at the same time, the fiber is being bent, the cable coils lengthened, shortened, or moved about in some way, often at a rapid rate.
Optical waveguide glass fibers are made from glass such as quartz glass or doped silica glass and are extremely small in diameter and hence weak in strength. Under normal conditions of use, such fibers may be subjected to considerable bending strain and tensile forces during the cable manufacturing process and while being pulled through ducts and otherwise bent, twisted, or manipulated during reeling or during installation. In the transmission of light signals through optical fiber cables, the signals are readily attenuated in response to relatively small distortions in the cable, such as those caused by the above stresses, sharp bends or roughness in the surface of the fiber to produce light signal distortions or attenuation at an unacceptable level.
To confine the optical signals inside the signal-transmitting fiber core, a glass or silica fiber core is coated with a glass cladding or an amorphous fluoropolymer coating always of a lower refractive index from that of the optical fiber. A coating is usually applied on top of the cladding. The coating on the glass cladding may optionally be a silicone, acrylic, polyimide, or other release agent and a polymer coating, which is usually a hard or soft polymer coating which is coated on the fiber from a melt or a solution of the polymer, or extruded onto the fiber. Many hard and soft plastic coatings have been tried and some of these have been applied in layers for varying purposes as disclosed in U.S.Pat. Nos. 4,113,350, 4,105,284, 4,380,367, 4,072,400, 3,930,103, 4,463,329, 4,099,837, and 4,642,265, of which the background discussion contained therein is hereby incorporated into this application. Another excellent discussion of optical fiber packaging and buffering is provided by a paper in the Bell System Technical Journal in Volume 54, No. 2, pages 245-262. Feb. 1975, by D. Gloge.
Similar considerations apply to plastic optical fibers also.
Loss of light from a fiber can also be induced by bending the fiber. This is a very important loss mechanism, as the presence or absence of this loss mechanism is determined by the fiber user. Improper cabling may produce small systematic perturbations to the fiber, causing an elevation of the initial loss. These losses, caused by small-amplitude (nanometer), high-spatial-frequency (millimeter) perturbations are called microbending losses. Even if cabled correctly, the fiber can be installed in tight-diameter bends (e.g., centimeters in radius) that also will raise the attenuation. This large-diameter bend loss is referred to as macrobend loss.
Increasing overall fiber diameter also decreases sensitivity, whereas increasing core diameter increases sensitivity, since the fiber will have a greater modal volume and tend toward more lossy modes The physical basis for the loss in the fiber is that a bend will change the optical path length of the fiber. The light propagating at the inside of the bend will travel a shorter distance than that travelling on the outside of the bend. To maintain coherence, the mode phase velocity must increase. But when a fiber bend is below the critical radius, this propagation velocity will exceed the speed of light and some of the light within the fiber is converted to higher-order modes and becomes radiative. The loss of these higher-order modes causes a gradual increase in attenuation.
A low-modulus coating generally will improve the insensitivity to random bends. However, this low-modulus coating may not produce sufficient mechanical protection for industrial handling, and a secondary, higher-modulus coating often is applied if a secondary coating is not applied. The lower-modulus coating typically is applied to a greater thickness, i.e., several hundred micrometers.
As the bend radius is decreased, a radius is reached at which the primary mode is lost at a given wavelength. As the bend radius is further decreased, the bend edge shifts to still shorter wavelengths. Thus, in single-mode fibers, the loss is wavelength dependent.
The bend sensitivity changes with the degree of power confinement, which is determined by the difference between the operating wavelength and single-mode cutoff wavelength. As the difference between these two parameters is decreased, and as the single-mode cutoff wavelength is shifted toward longer wavelengths, the optical loss caused by fiber bends decreases.
Much effort has been spent in the past to protect optical fibers from breaking, microbending, vibrating, excessive bending, compression, and other forms of movement. This has often resulted in coating or wrapping the fiber in various forms of polymer and metal layers as discussed above in an attempt to protect the fiber so as to permit movement commensurate with the application in which it is desired in order to use it without at the same time degrading the signal carried by the fiber by continuous flexing of the cable.
One means of protecting the optical fiber against problems caused by movement has been to house the fiber loosely in a hollow tube which may be coiled to allow lengthening and shortening of the cable without applying torsional strain to the fiber. Such hollow tubes, however, tend to have low crush resistance. Hollow tubes also allow the fiber to have free radial motion which causes fatigue in flexure and increases the allowable bend radius of the fiber. There is also a problem of wear and abrasion of the coatings of the fiber against the walls of the tube. During stripping of a cable in the termination process, it is also easier to damage a fiber if it is located off-center in the loosely-fitting hollow tube. Being loose in the tube the fiber is subject to torsion without support while being held by the stripper mechanism.
The cable of the invention provides an improved solution to these problems.